Precuring apparatus and method for reducing voltage required to electrostatically material to an arcuate surface

ABSTRACT

A precurl device for adjusting the curvature of the paper prior to being disposed on a transfer drum (48). The paper is fed along a path defined by a guide (296) to a nip between two precurl rollers (244) and (246). The durometer of the roller (246) is higher than the durometer of the roller (244), such that the roller (244) will deform at the nip between the rollers. The paper is fed to an attachment roller (198) that is adjacent the drum (48). A variable precurl device (312) is operable to vary the force on the roller (244) against the roller (246), to vary the amount of arcuate deformation imparted to the paper (146).

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent applicaton Ser. No.08/141,273, filed Dec. 6, 1993 and entitled "Buried Electrode Drum foran Electrophotographic Print Engine with Controlled Resistivity Layer"(Atty. Dkt. No. TRSY-21, 880), which is a continuation-in-part of U.S.patent application Ser. No. 07/954,786, filed Sep. 30, 1992 now U.S.Pat. No. 5,276,490, and entitled "Buried Electrode Drum for anElectrophotographic Print Engine" (Atty. Dkt. No. TRSY-21,072).

TECHNICAL HELD OF THE INVENTION

The present invention pertains in general to electrophotographic printengines, and more particular, to the feeding mechanism for feeding paperto an electrostatic drum or transfer belt.

BACKGROUND OF THE INVENTION

When utilizing electrostatic gripping on a transfer drum or belt, thevoltage is typically applied at such a level that adherence of the paperto the drum is adequate. However, if the voltage is reduced below acertain level, some difficulty exists in adhering the paper to the drumor transfer belt. This is due to the fact that the paper has a tendencyto lay flat, whereas the drum or transfer belt has an arcuate surface.Of course, after the paper has been on the drum for a sufficient amountof time, it will conform to the shape of the surface. Unfortunately,high speed copiers at present do not allow the paper to reside on thedrum for very long.

In electrophotographic equipment, it is necessary to provide variousmoving surfaces which are periodically charged to attract tonerparticles and discharged to allow the toner particles to be transferred.At present, three general approaches have been embodied in products inthe marketplace with respect to the drums. In a first method, theconventional insulating drum technology is one technology that grips thepaper for multiple transfers. A second method is the semi-conductivebelt that passes all the toner to the paper in a single step. The thirdtechnology is the single transfer to paper multi-pass charge, expose anddevelopment approach.

Each of the above approaches has advantages and disadvantages. Theconventional paper drum technology has superior image quality andtransfer efficiency. However, hardware complexity (eg., paper gripping,multiple coronas, etc.), media variability and drum resistivity add tothe cost and reduce the reliability of the equipment. By comparison, thesingle transfer paper-to-paper system that utilizes belts has anadvantage of simpler hardware and more reliable paper handling. However,it suffers from reduced system efficiency and the attendant problemswith belt tracking, belt fatigue and handling difficulties duringservice. Furthermore, it is difficult to implement the belt system tohandle multi-pass to paper configuration for improved efficiency andimage quality. The third technique, the single transfer-to-paper system,is operable to build the entire toner image on the photoconductor andthen transfer it. This technique offers simple paper handling, but atthe cost of complex processes with image quality limitations and therequirement that the photoconductor surface be as large as the largestimage.

SUMMARY OF THE INVENTION

The present invention disclosed and claimed herein comprises a paperfeed device for feeding paper onto a rotating image carrier. A directingdevice is provided for directing a sheet of paper along a defined path.A precurl device then deforms the sheet of paper to have an arcuatedeformation. A precurl control controls the amount of arcuatedeformation imparted to the paper by the precurl device. An attachmentdevice then attaches the paper to the image carrier after arcuatedeformation thereof by the precurl device.

In another aspect of the present invention, the image carrier has acurvature associated therewith that is in the same direction as thearcuate deformation of the paper. The precurl device is operable toprovide this arcuate deformation through two adjacent rollers having anip disposed therebetween. The nip is disposed along the paper path,with the durometer of the first roller being greater than the durometerof the second roller. This results in the second roller being deformedby the first roller, at least one of the first and second rollers beingdriven.

In yet another aspect of the present invention, the precurl control isoperable to impart a variable pressure to the nip such that thepredetermined pressure can be varied. This is done such that the firstroller has substantially no deformation associated therewith due to thevariable pressure applied to the rollers at the nip.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying Drawings in which:

FIG. 1 illustrates a perspective view of the buried electrode drum ofthe present invention;

FIG. 2. illustrates a selected cross section of the drum of FIG. 1;

FIG. 3 illustrates the interaction of the photoconductor drum and theburied electrode drum of the present invention;

FIG. 4 illustrates a cutaway view of the electrodes at the edge of thedrum;

FIGS. 5a and 5b illustrate alternate techniques for electrifying thesurface of the drum;

FIGS. 6a-6c illustrate the distributed resistance of the buriedelectrode drum of the present invention;

FIGS. 7a and 7b illustrate the arrangement of the electrifying rollersto the edge of the drum;

FIG. 8 illustrates a side view of a multi-pass-to-paperelectrophotographic print engine utilizing the buried electrode drum;

FIG. 9 illustrates a cross section of a single pass-to-paper printengine utilizing the varied electrode drum;

FIG. 10 illustrates an alternate embodiment of the overall constructionof the drum assembly;

FIG. 11 illustrates another embodiment wherein a resilient layer of theinsulating material is disposed over the aluminum core with electrodesdisposed on the surface thereof;

FIG. 12, illustrates another embodiment of the present invention whereinthe core of the drum is covered by an insulating layer with a conductinglayer disposed on the upper surface thereof;

FIG. 13 illustrates another embodiment of the transfer drum;

FIG. 14 illustrates another embodiment of the transfer drumconstruction;

FIG. 15 illustrates another embodiment of the transfer drumconstruction;

FIG. 16 illustrates another embodiment of the transfer drum;

FIG. 17 illustrates an embodiment illustrating the interdigitatedelectrodes described above with respect to FIG. 15;

FIG. 18 illustrates a detail of the physical layers in a section of theBED drum with the paper attached thereto;

FIG. 19 illustrates a diagrammatic view of the paper layer, the filmlayer and the uniform electrode layer;

FIG. 20 illustrates a schematic representation of the paper and filmlayers;

FIG. 21 illustrates a schematic diagram of the overall operation of thetransfer drum;

FIG. 22 illustrates a cross sectional diagram of the structure of FIG.19, when it passes under a photoconductor drum, which is in a dischargemode;

FIG. 23 illustrates another view of the spatial difference between thephotoconductor drum and the paper attach electrode disposed about theburied electrode drum;

FIG. 24 illustrates a plot of simulated voltage vs. time for anarbitrary section of paper as it travels around the drum 48 four timesin a four pass (i.e., color) print;

FIG. 25 illustrates a simulated voltage vs. time plot of a single pass;

FIG. 25a illustrates a graph of decay voltages;

FIG. 26 illustrates a simulated voltage vs. time plot of a four passoperation;

FIG. 27 illustrates a simulated voltage vs. time plot of a four passoperation;

FIG. 27a illustrates an alternate simulated voltage vs. time plot of afour pass operation utilizing Mylar;

FIG. 28 illustrates a simulated voltage versus time plot for anarbitrary section of paper as it travels around the drum four timesduring a four pass color print with no discharge before attack;

FIG. 29 illustrates the operation of FIG. 29 with discharge;

FIG. 30 illustrates a side-view of the overall electrophotographicprinter mechanism;

FIG. 31 illustrates a detail of the pre-curl device;

FIG. 31a illustrates a detail of the pre-curl operation for the pre-curlrollers;

FIGS. 32a and 32b illustrate devices to measure paper droop and curl;and

FIG. 33 illustrates a view of the pre-curl rollers.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is illustrated a perspective view of theburied electrode drum of the present invention. The buried electrodedrum is comprised of an inner core 10 that provides a rigid supportstructure. This inner core 10 is comprised of an aluminum tube core of athickness of approximately 2 millimeters (ram). The next outer layer iscomprised of a controlled durometer layer 12 which is approximately 2-3mms and fabricated from silicon foam or rubber. This is covered with anelectrode layer 14, comprised of a plurality of longitudinally disposedelectrodes 16, the electrodes being disposed a distance of 0.10 inchapart, center line to center line, approximately 0.1 mm. A controlledresistivity layer 18 is then disposed over the electrode layer to athickness of approximately 0.15 mm, which layer is fabricated fromcarbon filled polymer material.

Referring now to FIG. 2, there is illustrated a more detailedcross-sectional diagram of the buried electrode drum. It can be seenthat at the end of the buried electrode drum, the electrodes 16 withinelectrode layer 14 are disposed a predetermined distance apart. However,the portion of the electrodes 16, proximate to the ends of the drum oneither side thereof are "skewed" relative to the longitudinal axis ofthe drum. As will be described hereinbelow, this is utilized to allowaccess thereto.

Referring now to FIG. 3, there is illustrated a side view of the buriedelectrode drum illustrating its relationship with a photoconductor drum20. The photoconductor drum 20 is operable to have an image disposedthereon. In accordance with conventional techniques, a latent image isfirst disposed on the photoconductor drum 20 and then transferred to thesurface of the buried electrode drum in an electrostatic manner.Therefore, the appropriate voltage must be present on the surface at thenip between the photoconductor drum 20 and the buried electrode drum.This nip is defined by a reference numeral 22.

A roller electrode 24 is provided that is operable to contact the uppersurface of the buried electrode drum at the outer edge thereof, suchthat it is in contact with the controlled resistivity layer 18. Sincethe electrodes 16 are skewed, the portion of the electrode 16 that isproximate to the roller electrode 24 and the portion of the electrode 16that is proximate to the nip 22 on the longitudinal axis of thephotoconductor drum 20 are associated with the same electrode 16, aswill be described in more detail hereinbelow.

Referring now to FIG. 4, there is illustrated a cutaway view of theburied electrode drum. It can be seen that the buried electrodes 16 aretypically formed by etching a pattern on the outer surface of thecontrolled durometer layer 12. Typically, the electrodes 16 areinitially formed by disposing a layer of thin, insulative polymer, suchas mylar, over the surface of the controlled durometer layer 12. Anelectrode structure is then bonded or deposited on the surface of themylar layer. In the bonded configuration, the electrode pattern ispredetermined and disposed in a single sheet on the Mylar. In thedeposited configuration, a layer of insulative material is disposed downand then patterned and etched to form the electrode structure. Althougha series of parallel lines is illustrated, it should be understood thatany pattern could be utilized to give the appropriate voltage profile,as will be described in more detail hereinbelow.

Referring now to FIGS. 5a and 5b, there are illustrated two techniquesfor contacting the electrodes. In FIG. 5a, a roller electrode isutilized comprising a cylindrical roller 24 that is pivoted on an axle26. A voltage V is disposed through a line 28 to contact the roller 24.The roller 24 is disposed on the edge of the buried electrode drum suchthat a portion of it contacts the upper surface of the controlledresistivity layer 18 and forms a nip 30 therewith. At the nip 30, aconductive path is formed from the outer surface of the roller electrode24 through the controlled resistivity layer 18 to electrode 16 in theelectrode layer 14. In this manner, a conductive path is formed. Theelectrodes 16 in the electrode layer 14, as will be describedhereinbelow, are operable to provide a low conductivity path along thelongitudinal axis of the buried electrode drum to evenly distribute thevoltage along the longitudinal axis.

FIG. 5b illustrates a configuration utilizing a brush 32. The brush 32is connected through the voltage V through a line 34 and has conductivebristles 36 disposed on one surface thereof for contacting the outersurface of the control resistivity layer 18 on the edge of the buriedelectrode drum. The bristles 36 conduct current to the surface of thecontrolled resistivity layer 18 and therethrough to the electrodes 16 inthe electrode layer 14. This operates identical to the system of FIG.5a, in that the electrode 16 in the electrode layer 14 distributes thevoltage along the longitudinal axis of the buried electrode drum.

Referring now to FIGS. 6a-6c, the distribution of voltage along thesurface of the electrode layer 14 will be described in more detail. Theburied electrode drum is illustrated in a planar view with the electrodelayer "unwrapped" from the controlled durometer layer 12 forsimplification purposes. Along the length of the controlled resistivitylayer 18 are disposed three electrode rollers, an electrode roller 40connected to the positive voltage V, an electrode roller 42 connected toa ground potential and an electrode roller 44 connected to a groundpotential. The electrode roller 40 is operable to dispose a voltage V onthe electrode directly therebeneath, which voltage is conducted alongthe longitudinal axis of the drum at the portion of the controlledresistivity layer 18 overlying the electrode 16 having the highestvoltage thereon. Since the electrode rollers 42 and 44 have a groundpotential, current will flow through the controlled resistivity layer 18to each of the electrode rollers 42 and 44 with a correspondingpotential drop, which potential drop decreases in a substantially linearmanner. However, at each electrode disposed between the roller 40 andthe rollers 42 and 44, the potential at that electrode 16 will besubstantially the same along the longitudinal axis of the buriedelectrode drum. In this configuration, therefore, the electrode roller40 disposed at the edge of the buried electrode drum is operable to forma potential at the edge of the buried electrode drum that is reflectedalong the surface of the buried electrode drum in accordance with thepattern formed by the underlying electrode 16. Therefore, the rollerelectrode 40, in conjunction with the electrode 16, act as individualactivatable charging devices, which devices can be arrayed around thedrum merely by providing additional electrode rollers at variouspotentials, although only one voltage profile is illustrated, manysegments could be formed to provide any number of different voltageprofiles. Additionally, local extremum voltages occur between electrodestrips 16 and overall extremum voltages occur between rollers 40, 42 and44.

FIG. 6b illustrates the potential along the length of the controlledresistivity layer 18. It can be seen that the highest potential is atthe electrode 16 underlying the electrode roller 40, since this is thehighest potential. Each adjacent electrode 16 has a decreasing potentialdisposed thereon, with the potential decreasing down to a zero voltageat each of the electrode rollers 42 and 44. The voltage profile shown inFIG. 6b shows that there is some lower voltage disposed between the twoelectrodes, due to the resistivity of the controlled resistivity layer18.

FIG. 6c illustrates a detailed view of the electrode roller 40 and theresistance associated therewith. There is a distributed resistancedirectly from the electrode roller 40 to the one of the electrodes 16directly therebeneath. A second distributive resistance exists betweenthe electrode roller 40 and the adjacent electrodes 16. However, each ofthe adjacent electrodes 16 also has a resistance from the surfacethereof upward to the upper surface of the controlled resistivity layer18. Since the resistance along the longitudinal axis of the buriedelectrode drum with respect to each of the electrodes 16 is minimal, thepotential at the surface of the controlled resistivity layer 18overlying each of the electrodes 16 will be substantially the same. Itis only necessary for a resistive path to be established between thesurface of the roller 40 and each of the electrodes. This current pathis then transmitted along the electrode 16 to the upper surface of thecontrolled resistivity layer 18 in accordance with the pattern formed byburied electrodes 16.

Referring now to FIGS. 7a and 7b, there are illustrated perspectiveviews of two embodiments for configuring the rollers. In FIG. 7a, theburied electrode drum, referred to by a reference numeral 48, has tworollers 50 and 52 disposed at the edges thereof and a predetermineddistance apart. The distance between the rollers 50 and 52 is a portionof the buried electrode drum 48 that contacts the photoconductor drum. Avoltage V is disposed on each of the rollers 50 and 52 such that thevoltage on the surface of the drum 48 is substantially equal over thatrange. A brush 54 is disposed on substantially the remaining portion ofthe circumference at the edge of the drum 48 such that conductivebristles contact all of the remaining surface at the edge of the drum48. The electrode brush 54 is connected through a multiplexed switch 56to either a voltage V on a line 58 or a ground potential on a line 60.The switch 56 is operable to switch between these two lines 58 and 60.In this configuration, one mode could be provided wherein the drum 48was utilized as a transfer drum such that multiple images could bedisposed on the drum in a multi-color process. However, when transfer isto occur, the switch 56 selects the ground potential 60 such that Whenthe drum rotates past the electrode roller 52, the voltage is reduced toground potential at the electrodes 16 that underlie the brush 54.

FIG. 7b illustrates the drum 48 and rollers 50 and 52 for disposing thepositive voltage therebetween.. However, rather than a brush 54 that isdisposed around the remaining portion at the edge of the drum 48, twoground potential electrode rollers 62 and 64 are provided, having atransfer region disposed therebetween. Therefore, an image disposed onthe buried electrode drum 48 can be removed from the portion of the linebetween rollers 62 and (34, since this region is at a ground potential.

Referring now to FIG. 8, there is illustrated a side view of amulti-pass-to-paper print engine. The print engine includes an imagingdevice 68 that is operable to generate a latent image on the surface ofthe PC drum 20. The PC drum 20 is disposed adjacent the buried electrodedrum 48 with the contact thereof provided at the nip 22. Supportingbrackets [not shown] provide sufficient alignment and pressure to formthe nip 22 with the correct pressure and positioning. The nip 22 isformed substantially midway between the rollers 50 and 52, which rollers50 and 52 are disposed at the voltage V. A scorotron 70 is provided forcharging the surface of the photoconductor drum 20, with three tonermodules, 72, 74 and 76 provided for a three-color system, this beingconventional. Each of the toner modules 72, 74 and 76, are disposedaround the periphery of the photoconductor drum 20 and are operable tointroduce toner particles to the surface of the photoconductor drum 20which, when a latent image passes thereby, picks up the toner particles.Each of the toner modules 72-76 is movable relative to the surface ofthe photoconductor drum 20. A fourth toner module 78 is provided forallowing black and white operation and also provides a fourth color forfour color printing. Each of the toner modules 72-78 has a reservoirassociated therewith for containing toner. A cleaning blade 80 isprovided for cleaning excess toner from the surface of thephotoconductor drum 20 after transfer thereof to the buried electrodedrum 48. In operation, a three color system requires three exposures andthree transfers after development of the exposed latent images.Furthermore, the modules 72-76 are connected together as a single modulefor ease of use.

The buried electrode drum 48 has two rollers 53 and 54 disposed oneither side of a pick up region, which rollers 53 and 54 are disposed atthe positive potential V by switch 56 during the transfer operation. Acleaning blade 84 and waste container 86 are provided on a cam operatedmechanism 88 such that cleaning blade 84 can be moved away from thesurface of the buried electrode drum 48 during the initial transferprocess. In the first transfer step, paper (or similar transfer medium)is disposed on the surface of the buried electrode drum 48 and thesurface of drum 48 disposed at the positive potential V, and also forthe second and third pass. After the third pass, the now completemulti-layer image will have been transferred onto the paper on thesurface of the buried electrode drum 48.

The paper is transferred from a supply reservoir 88 through a nip formedby two rollers 90 and 92. The paper is then transferred to a feedmechanism 94 and into adjacent contact with the surface of the drum 48prior to the first transfer step wherein the first layer of themulti-layer image is formed. After the last layer of the multi-layerimage is formed, the rollers 53 and 54 are disposed at ground potentialand then the paper and multi-layer image are then rotated around to astripper mechanism 96 between rollers 53 and 54. The stripper mechanism96 is operable to strip the paper from the drum 48, this being aconventional mechanism. The stripped paper is then fed to a fuser 100.Fuser 100 is operable to fuse the image in between two fuse rollers 102and 104, one of which is disposed at an elevated temperature for thispurpose. After the fusing operation, the paper is feed to the nip of tworollers 106 and 108, for transfer to a holding plate 110, or to the nipbetween two rollers 112 and 114 to be routed along a paper path 116 to aholding plate 118.

Referring now to FIG. 9, there is illustrated a side view of anintermediate transfer print engine. In this system, the three layers ofthe image are first disposed on the buried electrode drum 48 and then,after formation thereof, transferred to the paper. Initially, thesurface of the drum is disposed at a positive potential by rollers 50and 52 in the region between rollers 50 and 52. During the first pass,the first exposure is made, toner from one of the toner modules disposedon the latent image and then the latent image transferred to the actualsurface of the buried electrode drum 48. During the second pass, a thirdtoner is utilized to form a latent image and this image transferred tothe drum 48. During the third pass, the third layer of the image isformed as a latent image using the second toner, which latent image isthen transferred over the previous two images on the drum 48 to form thecomplete multi-layer image.

After the image is formed, paper is fed from the tray 88 through the nipbetween rollers 90 and 92 along a paper path 124 between a nip formed bya roller 126 and the drum 48. The roller 126 is moved into contact withthe drum 48 by a cam operation. The paper is moved adjacent to the drum48 and thereafter into the fuser 100. During transfer of the image tothe paper, two rollers 130 and 132 are provided on either side of thenip formed between the roller 126 and the drum 48. These two rollers 130and 132 are operable to be disposed at a positive voltage by multiplexedswitches 134 and 136 during the initial image formation procedure.During transfer to the paper, the rollers 130 and 132 are disposed at aground voltage with the switches 134 and 136. However, it should also beunderstood that these voltages could be a negative voltage to actuallyrepulse the image from the surface of the drum 48.

Referring now to FIG. 10, there is illustrated an alternate embodimentof the overall construction of the drum assembly. The aluminum supportlayer 10 comprises the conductive layer in this embodiment, whichaluminum core 10 is attached to a voltage supply 140. The voltage supply140 provides the gripping and transfer function, as will be describedhereinbelow. The voltage supply 140 is applied such that it provides auniform application of the voltage from the voltage supply 140 to theunderside of a resilient layer 142. The resilient layer 142 is aconductive resilient layer with a volume resistivity under 10¹⁰ Ohm-cm.The layer 142 is fabricated from carbon filled elastomer or materialsuch as butadiene acrylonitrile. The thickness of the layer 142 isapproximately 3 mm. Overlying the resilient layer 142 is a controlledresistivity layer 144 which is composed of a thin dielectric layer ofmaterial with a thickness of between 50 and 100 microns. The layer 144has a non-linear relationship between the discharge (or relaxation) timeand the applied voltage such that, as the voltage increases, thedischarge time changes as a function thereof. Overlying the layer 144 isa layer of support material 146, which is typically paper. Thephotoconductor drum 20 contacts the paper 146.

Referring now to FIG. 11, there is illustrated another embodimentwherein a resilient layer 148 of an insulating material comprised ofNeoprene is disposed over the aluminum core 10 with electrodes 14disposed on the surface thereof. The electrodes 14 are disposed in alayer, each of the electrodes 14 comprised of an array of conductorsseparated by a predetermined distance. The conductors 14 are covered bya controlled resistivity layer 150, similar to the controlledresistivity layer 144 in FIG. 10, the gripping layer 150 covered by acontrolled resistivity layer with a surface resistivity of between 10⁶-10¹⁰ Ohm/sq. The controlled resistivity layer 152 is fabricated fromFLEX 200 and has a thickness of 75 microns. This is covered by thesupport layer 146. The distance between the electrodes 14 is defined bythe following equation: ##EQU1## where V_(d) is the allowable voltagedroop between electrodes,

i_(d) is the toner transfer current;

s is the spacing of the electrodes;

r is the sum of the surface resistivity and volume resistance of thelayer 150, and

w is the overall length of the electrode, which is nominally the widthof the drum 10.

The voltage source 140 is connected to the electrodes 14, as describedhereinabove, wherein a conductive brush or roller directly contacts anexposed portion of the electrodes on the edge of the drum or conductsthrough the upper conductive layers.

Referring now to FIG. 12 there is illustrated another embodiment of thepresent invention wherein the core of the drum 10 is covered by aninsulating layer 154 of a thickness 3ram and of a material utilizingNeoprene, with a conducting layer 156 disposed on the upper surfacethereof. The conductive layer 156 is connected to the voltage source140. This layer provides the advantage of separating the electricalcharacteristics of the material from the mechanical characteristics.This is covered by an insulative layer 158, similar to the grippinglayer 144, with the paper 146 disposed on the upper surface thereof.

Referring now to FIG. 13, there is illustrated another embodiment of thetransfer drum. A voltage source 160 is connected to the core 10 and thecore 10 then has a conductive resilient layer 162 disposed on thesurface thereof. The electrodes 14 are disposed in a layer on the uppersurface of the layer 162 with the voltage source 164 connected theretothrough a conductive brush or such. The voltage supplies 160 and 164 areused to establish the uniform voltage on the underside of the resilientconductive layer 162 and a voltage profile on the top side. The benefitof this configuration is to provide a variable surface potential whilemaintaining a uniform gripping voltage source. A gripping layer 168 isdisposed on the upper surface of the electrodes 14, similar to thegripping layer 158, which is then covered by the paper 146.Additionally, it is noted that by applying the voltage 164 that isdifferent than the voltage of supply 160 (perhaps even 0), a voltageprofile with a voltage minimum will be obtained at the entrance to thenip. This will reduce the pre-nip discharge for multiple transferoperation. This voltage minimum characteristic is also shown in FIG. 6a.

Referring now to FIG. 14, there is illustrated another embodiment of thetransfer drum construction. In this configuration, an insulating core170 is provided, similar to the dimension of the core 10 but fabricatedfrom insulating material such as polycarbonate. The electrode layer withelectrodes 14 is then disposed on the surface of the insulating core 170and the voltage source 140 connected thereto. A conducting resilientlayer 172 is disposed on the surface of the electrodes 14 to a thicknessof 3 mm and fabricated from butylacrylonitrile. A gripping layer 174,similar to the gripping layer 144 is disposed on top of the resilientlayer 172, with the paper 146 disposed on the upper surface thereof.

Referring now to FIG. 15, there is illustrated another embodiment of thetransfer drum construction. The conducting layer 156 in FIG. 11 isremoved such that a layer of interdigitated electrodes 176 can beutilized between the gripping layer 152 and the resilient layer 148.This resilient layer, as described above, is an insulating layer. Thevoltage source 140 is connected to the electrodes 176. Theinterdigitated electrodes increase the value of w in Equation 1, thusallowing a much higher value of r in Equation 1. The interdigitatedelectrodes are illustrated below in FIG. 17.

Referring now to FIG. 16, there is illustrated another embodiment of thepresent invention. The core 10 has disposed thereon a first resilientlayer 180, covered by the electrode layer having electrodes 14 disposedtherein. The electrodes 14 are connected to a voltage source 140 throughconductive brushes or the such. A second resilient layer 182 is disposedover the electrodes 14 with the paper 146 disposed on the surfacethereof. The layer 180 can be a resilient layer that is resistive orinsulative. The resilient layer 182 is resistive with a resistivity ofless than 10¹⁰ Ohms/cm. The advantage provided by this configuration isthat the physical effects (i.e., nip pressure variations) of theelectrode layer are reduced by enclosing the electrodes 14 in tworesilient layers 180 and 182.

Referring now to FIG. 17, there is illustrated an embodimentillustrating the interdigitated electrodes described above with respectto FIG. 15. The interdigitated electrodes each have a plurality oflongitudinal arms 184 with extended or interdigitated electrodes 186 and188 extending from either side thereof. Adjacent electrodes will havethe interdigitated arms or electrodes 186 and 188 offset along thelongitudinal arm 184 such that they will interdigitate with each other,thereby effectively increasing apparent "w" of Equation 1, such that thecontrolled resistivity layer can be at a higher resistivity to the pointthat it can be eliminated.

Referring now to FIG. 18, there is illustrated a detail of the physicallayers in a section of the BED drum 48 with the paper 146 attachedthereto. An electrode strip 190 is disposed between a controlleddurometer layer 192 and a controlled resistivity layer 194. Thecontrolled durometer layer 192 represents the resilient layer 142 inFIG. 10 and subsequent figures. The controlled resistivity layer 194represents the gripping layer 144 in FIG. 10. The controlled durometerlayer 192 is disposed between the electrode strip layer 190 and thealuminum drum 10, the electrode strip layer 190 either comprising aplurality of electrodes in strips, as described above, or a singlecontinuous layer.

Referring now to FIG. 19, there is illustrated a diagrammatic view ofthe paper layer 146, the film layer 194 and the uniform electrode 196layer, which comprises the electrode strip layer 190. A paper attachelectrode 198 is provided, which is operable to contact the paper anddispose a potential thereon which, in the preferred embodiment, isground. At the point the electrode 198 contacts the paper 146, a nip 200is formed.

Referring now to FIG. 20, there is illustrated a schematicrepresentation of the layers 146, 174 and 196. A first capacitor 202,labelled C_(P), represents a paper layer 146, with a parallel resistor204 labelled R_(P). The film layer 194 is represented by a capacitor 206labelled C_(F), with a resistor 208 disposed in parallel therewith.,labelled R_(F). The electrode layer 196 is represented by a resistance 210 labelled R_(E), which goes to a transfer/attach power supply.

Referring now to FIG. 21, them is illustrated a schematic diagram of asimulator circuit capable of simulating the overall operation of thetransfer drum 48. The schematic representation shows a switch 212 thatis labelled K_(P) which is the charge relay, which is operable toconnect the upper surface of a paper layer 146, represented by thecapacitor 206 and resistor 204, to ground when the switch 212 is closed.A attach/transfer voltage source 214 is provided, having the positivevoltage terminal thereof connected to the most distal side of resistor210 and essentially to the uniform electrode layer 197. The other sideof the supply 214 is connected to ground. A switch 216 is provided whichis labelled K_(F), which is operable to connect the positive side of thesupply 214 to the top of the film layer 194. This is a dischargeoperation that will be described in more detail hereinbelow.

When paper is first presented to the drum in the nip 200 for attachment,the charge distribution of FIG. 19 is illustrated wherein positivecharges are attracted to the upper surface of the paper and negativecharges attracted to the lower surface thereof. Similarly, the positivecharges are attracted to the upper surface of the film layer 194 andnegative charges attracted to the lower surface thereof, with positivecharges attracted to the surface of the uniform electrode 196. Thisresults in mirror images of equal and opposite charges formed at eachinterface boundary between the various layers 146, 194 and 196. With thedielectric layers, layers 146 and 194, most of these charges are justbelow the surfaces of the respective layers and cannot cross theinterface boundary between the film. However, the charges are stronglyattracted to each other and provide the attractive force which holds thepaper on the drum. This attractive force is normal to the surface of thedrum and directly bonds the paper layer 146 to the drum in thatdirection. Additionally, this normal force is operable for generatingthe frictional forces that secure the paper to the drum in the remainingtwo axis, preventing paper slip. The source charge for the paperattachment is the attach/transfer supply 214. The switch 212 representsthe paper attach electrode 198.

When a selection of paper enters the nip 200, the composite capacitorformed by the paper and film layers is charged in a manner similar tothe charging of C_(P) and C_(F) as illustrated in FIG. 21 when the relayK_(P) is closed. If the dwell time of a section of paper in the attachnip 200 is sufficiently long relative to the time constant of theresistor 210 (R_(E)) and the series connected pair capacitor C_(P) andC_(F), this composite capacitor will charge to a voltage very nearlyequal to that of the attach/transfer supply 214. Fully charging thepaper film composite capacitor results in the maximum transfer of chargeand therefore the generation of the maximum attractive or bonding forceof the paper to the drum assembly.

After the paper leaves the attach nip 200, the capacitance that isassociated with the paper and film layers begins to discharge. The paperlayer then discharges at a rate determined by its dielectric content andvolume resistivity, with near complete discharge, i.e., to only a smallvoltage across the paper, occurring in less than 300 milliseconds. Thisdischarge is similar to the discharge behavior of C_(P) and R_(P) inFIG. 21. The film layer also discharges at a rate determined by itsdielectric constant and the volume resistivity (and other factors), butthe time required is much longer than that of the paper. The film layer194 may require more than 200 seconds for near complete discharge, anddoes so in a manner that is similar to the discharge characteristics ofC_(F) and R_(F) in FIG. 4.

The larger discharge time of the film layer 94 accounts for the abilityof the transfer drum to grip paper much longer than the discharge timeof the paper would indicate. Even though the voltage across the papercollapses relatively quickly, the trapped charges that were induced atthe paper's surface are trapped at the paper surface by the residualvoltage on the film layer. The trapped charges eventually migrate backinto the bulk of the paper, but only after the film layer 194 hasdischarged significantly.

Because of the large discharge time of the film layer 194, somemechanism to discharge the film completely between successive paperattach intervals is required. This function is simulated by the relayK_(F) in FIG. 21. The actual discharge mechanism is very similar to theattach electrode 198 in FIG. 19, but the discharge electrode is held atthe same potential as the electrode layer 196 to facilitate discharge.The discharge electrode is physically located upstream of the paperattach area and is in contact with the drum 48 only during the paperattach operation.

With further reference to FIG. 21, the operation of the layeredstructure of FIG. 18 will be described in more detail as to its effecton the paper gripping operation. By way of the example, in the casewhere a very resistant paper or transparency material is utilized, theresistance of resistor 210 (R_(E)) is much less than the resistance ofthe paper R_(P), and the resistance of resistor 210 (R_(E)) is much lessthan resistor R_(F). The composite capacitor will charge to the appliedvoltage with the time constant R_(E) C_(EQ), where: ##EQU2## If the timeconstant R_(E), C_(EQ) is much less than the time constant T_(N), whereT_(N) is equal to the time that a section of paper is present in theattachment 200, then the voltage across the capacitor will very nearlyreach the magnitude of the attach/transfer voltage of voltage supply 214(V_(A)). The voltages across each of the components of the composite.capacitor, C_(P) and C_(F), are given by:

    V.sub.CP =V.sub.A (C.sub.F /(C.sub.P +C.sub.F))            (3)

    V.sub.CF =V.sub.A (C.sub.p /(C.sub.p +C.sub.F))            (4)

For the actual paper and film layer of the drum, the analogous equationsare:

    V.sub.P =V.sub.A (ε.sub.F /((t.sub.F /t.sub.P)ε.sub.P +ε.sub.F)=V.sub.CP                                (5)

    V.sub.F =V.sub.A (ε.sub.P /((t.sub.P /t.sub.F)ε.sub.F +ε.sub.F)=V.sub.CF                                (6)

where:

ε_(P) =dielectric constant of the paper

ε_(F) =dielectric constant of the film

t_(P) =thickness of the paper

t_(P) =thickness of the film

The magnitude of the gripping force is directly proportional to theamount of charge trapped at the paper/film interface and, to maximizeit, the composite capacitance, C_(EQ), must be as large as possible.From Equation 2, it can be seen that, for a given paper, the largestvalue that the composite capacitance can have is C_(P). This occurs whenC_(F) is much greater than C_(P). Therefore, Equation 2 can be rewrittenas:

    C.sub.EQ =Aε.sub.p ε.sub.F /(t.sub.F ε.sub.P +t.sub.F ε.sub.P)                                 (7)

where A=area of the paper section in for a given paper with a dielectricconstant of ε_(P) and thickness t_(P), C_(EQ) approaches a value ofC_(P) if the dielectric constant of the film is much greater than thedielectric constant of the paper, or the thickness of the film is muchsmaller than the thickness of the paper. Under these conditions,Equations 5 and 6 indicate that, during attach, most of the voltage willbe developed across the paper, a desirable condition for good gripping.

In the case where the resistance R_(E) is substantially equal to theresistance of the paper R_(P), i.e., for very low resistance paper, theequations will differ somewhat. When the section of paper 146 enters thenip 200, both C_(P), and C_(F) will act as short circuits. However, ifC_(P) is much less than C_(F), C_(P) begins charging to:

    V.sub.p =V.sub.A (R.sub.P /(R.sub.P +R.sub.E))             (8)

with a time constant of:

    (R.sub.E R.sub.P /(R.sub.E +R.sub.P)) C.sub.P              (9)

Then, if the time constant R_(E) C_(F) is much less than T_(N), andR_(P) C_(F) is much less than T_(N), C_(P) will charge to V_(A) with atime constant (R_(E) +R_(P)) C_(F) while C_(P), completely dischargesthrough R_(P). Equation 8 indicates that, to maximize the voltage acrossthe paper, R_(E) should be selected such that R_(E) is much less thanR_(P). Additionally, it is equally important that C_(F) be selected suchthat C_(P) is much less than C_(F).

For the case where the resistance of the paper is much less than theresistance of the electrode layer 196 and much less than the resistanceof the film, Equation 8 shows that very little voltage will be developedacross the paper. Thus, only a very small gripping force will begenerated.

After the paper 146 is gripped onto the upper surface of the film layer194, toner must then be transferred from the photoconductor to thepaper. Since toner transfer efficiency is a function of applied voltagein the transfer nip, it is desirable that the dielectric composed of thepaper and film layers have no memory of the attach operation (i.e.,these layers would be fully discharged) as a section of the paper 146enters the transfer nip, thus allowing complete and independent controlof the transfer nip voltage. However, if the paper and film were fullydischarged, they would not be electrostatically attached to the drum, anundesirable situation.

Referring now to FIG. 22, there is illustrated a cross sectional diagramof the structure of FIG. 19, when it passes under a photoconductor drum218 which is in a discharge mode, i.e., there is ground potentialapplied thereto. Toner particles 222 are disposed on the photoconductordrum 218 and have a negative charge placed thereon. This is aconventional transfer operation. When the paper 146 passes under thephotoconductor drum 218, a transfer nip 220 is formed. Since theelectrode layer 196 is a uniform electrode, the voltage of the layer 196is that of the attach/transfer voltage source 214. This will result in astrong force of attraction at the film and paper interface, representedby a reference numeral 224.

Referring now to FIG. 23, there is illustrated another view of thespatial difference between the photoconductor drum 218 and the paperattach electrode 20 disposed about the buried electrode drum 48. It canbe seen that the distance between the paper attach electrode 20 and thephotoconductor 218 requires a time T_(ATT) for the paper to move fromthe paper attach nip 200 to the transfer nip 220. Additionally, the timefor the paper to traverse the entire circumference of the drum 48 is thetime T_(REV). Additionally, a discharge roller 201 is provided which isconnected to ground for completely discharging the surface.

Referring now to FIG. 24, there is illustrated a simulated voltageversus time plot for an arbitrary section of paper as it travels aroundthe drum 48 four times in a four pass (i.e., color) print. The firsttransition to zero potential is caused by the paper attach electrode 20contacting the drum and the paper passing into the paper attach nip 200,this represented by the relay 212 (K_(P)) in FIG. 21 closing. This isrepresented by a point 223. The paper will then move to the tonertransfer nip 220, where the voltage will again go to a zero potential,as represented by a point 225, the time difference between points 223and 225 being T_(ATT). This will be a toner transfer point. Then thepaper traverses around the drum and the voltage will increase to ahigher voltage level (relative to ground potential) at a point 226 aftertime T_(REV), at which time the paper will again arrive at the tonertransfer nip 220 and the potential will again go to zero as representedby a point 228. Of course, the paper attach electrode 20 has beenremoved after the last portion of the paper was attached to the drum 48,in the first pass, this being a single pass. This will continue forthree more passes up to a point 230. Each of the transitions at thetransfer nip 220 are also represented by closure of the relay 2 14 inthe simulation of FIG. 21. Because the surface of the photoconductordrum 218 is either discharged or at a low potential (relative to theapplied transfer voltage of source 214), the photoconductor drum 218performs much like the attach electrode 20 in an electrical sense.Although not discussed or shown in detail, the voltage of source 214 isstepped up slightly for each successive toner transfer to account forthe thickness of the previous toner layer, this being a conventionaloperation.

The surface of the paper is held at a zero potential for the entire timethat it is in either the paper attach nip 200 or the transfer nip 220.During this time, the paper and film composite capacitor (C_(EQ))becomes very nearly charged to the full potential of the attach/transfersource 214. Upon leaving either of these nips, the capacitance C_(EQ)begins to discharge. The first portion of the discharge occurs betweenpoints 223 and 225 and is quite rapid, approximately 170 milliseconds,this due primarily to the paper discharging. This is equivalent to thecapacitance C_(P) discharging through the resistance R_(P) and isillustrated in more detail in FIG. 25. In the second portion of thecurve between points 225 and 228, and subsequent passes to point 230, itcan be seen that the discharge is quite slow, wherein only a partialdischarge is apparent. This is equivalent to the capacitance C_(F)discharging through the resistance R_(F). In the preferred embodiment,the voltage on the electrode layer 196 is held at a constant voltage of1500 volts for the curves of FIG. 24 and FIG. 25.

The voltage available for transfer of toner is the difference betweenthe voltage at the surface of the paper and ground potential, justbefore the paper enters the transfer nip 220. Thus, for a constantvoltage on drum 48, the amount that the film layer discharges betweeneach successive toner transfer pass (i.e., each revolution of the drum4.8) determines the amount of voltage available for toner transfer.

The amount of time available for the paper/film discharge after thepaper is attached is the time T_(ATT) for the first layer of toner. Theamount of time available for the paper/film discharge is the timeT_(REV), as illustrated in FIG. 23. This time is required for thesubsequent layers of toner and, therefore, the voltage across the filmlayer 194 must not discharge to a level too low to maintain attraction,but it must discharge sufficiently to allow a voltage difference at thetransfer nip 220. The film layer 194 should have a discharge timeconstant approximately equal to T_(ATT) to minimize the effect of theresidual voltage on the film layer during transfer of the first layer oftoner, and yet reserve sufficient potential across the film to maintaingripping of the paper (if R_(F) C_(F) is much less than T_(ATT),gripping cannot be maintained). However, for the configurationillustrated in FIG. 23, T_(ATT) =T_(REV) /4 and gripping must bemaintained for at least as long as T_(REV).

This relationship suggests that the film layer should have a voltagedependant discharge time constant; that is, the RC time constant (orrelaxation time constant) of the film should be small for highpotentials and large for low potentials. A voltage dependentcharacteristic of this type would allow large potentials to be used forpaper attach and toner transfer and allow a small but sufficientresidual potential in the film layer for paper gripping maintenance.Because the residual would be small, effects of previous paper attachand toner transfer operations on those subsequent thereto would beminimized.

It is well known that the discharge time constant or RC time constantfor a capacitor or film layer is characterized by the equation:

    V=V.sub.o *e-(t/RC)                                        (10)

where:

V is the voltage, across a film,

V_(o) is the initial voltage,,

t is time,

C is the capacitance of the film, and

R is the resistance of the film.

The characteristic discharge time is that time that equals the productof RC, and so the exponential term is unity. Specifically the dischargetime is given by the equation:

    t=RC                                                       (11)

It is of particular importance that in the case of a preferred grippinglayer the characteristics of the film do not behave according to theabove equation. Specifically, the behavior of the film discharge timeconstant is a function of voltage as well as R and C, or morespecifically R and/or C are a function of voltage and not constant forthe film material. And more specifically, for the improved performanceof the gripping layer, the discharge time for the film decreases withincreasing voltage:

    V=V.sub.o *e-(t/f(R,C,V))                                  (12)

In this case, the exponent is a function that is dependent on V. This"nonlinear" behavior is important for the gripping layer to decaysufficient for transfer voltage and yet retain sufficient voltage forgripping. This is shown graphically in the graph of FIG. 25a. Note thatthe preferred nonlinear characteristic in the nonlinear decay curve isreflected in quicker initial discharge characteristics for good transferand then a slowing to a higher value for improved gripping.

Tables 1 and 2 illustrate discharge characteristics for two films whosedielectric contents are very nearly equal. The film associated withTable 1 is an extruded tube of Elf Atochem Kynar Flex 2800, aproprietary copolymer formed using polyvinylidene fluoride (PVDF) andhexafluoropropolene (HFP). The average wall thickness was approximately4 mils. The manufacturer's specification for the dielectric for the filmis (9.4-10.6) ε_(o). The volume resistivity is specified as 2.2×10¹⁴Ohm-centimeters. The film associated with Table 2 was obtained fromDuPont as cast 8.5"×11" sheets of Tedlar (TST20SG4), a polyvinylfluoride (PVF) polymer. The average thickness was approximately 2 mils.The manufacture's specifications for the dielectric constant of the filmis (8-9) ε_(o). The volume resistivity is specified as 1.8×10¹⁴Ohm-centimeters.

                  TABLE 1                                                         ______________________________________                                        INITIAL      SECONDS FOR DISCHARGE TO                                         VOLTAGE V    3/4V   V/2       0.37V V/4                                       ______________________________________                                        1600         1.4    4.9       10.3  22.1                                      1400         1.7    5.1       12.8  27.3                                      1200         2.2    8.1       16.6  37.6                                      1000         2.9    9.6       19.8  41.0                                       800         5.3    16.8      32.1  54.9                                       600         8.2    26.4      45.9  78.9                                       400         12.4   39.4      64.5  105.8                                      200         13.3   43.9      74.9  123.8                                     ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        INITIAL      SECONDS FOR DISCHARGE TO                                         VOLTAGE V    3/4V   V/2       0.37V V/4                                       ______________________________________                                        1600         1.4    13.4      22.8  39.4                                      1400         6.0    19.1      29.7  49.4                                      1200         7.2    21.3      36.1  59.6                                      1000         8.8    27.7      45.7  74.7                                       800         10.9   33.1      54.7  87.5                                       600         13.5   40.3      65.0  103.8                                      400         16.7   48.6      78.3  123.8                                      200         20.3   59.8      95.6  147.8                                     ______________________________________                                    

The discharge time constant (R_(F) C_(F)) measured for low startingvoltages are very nearly equal and are in agreement with themanufacturers stated values for dielectric constant and volumeresistivity. Each of the two films exhibit the voltage dependentdischarge time constant. By comparing the discharge times in the 3/4 Vcolumn, it can be seen that the film associated with Table 1 dischargesfaster at high voltages than does the film of Table 2. The response forTable 1 is illustrated in FIG. 26 and the response for the film of Table2 is illustrated in FIG. 27. FIG. 27a illustrates a response for a filmsuch as Mylar, which response illustrates that insufficient voltage isavailable for subsequent (multiple) passes. Film voltage is held at aconstant 2200 volts for each type. The discharge characteristics of FIG.26 are preferred. In the film of FIG. 27a, the film was manufactured byApollo as a transparency material. Its chemical and electricalproperties are unknown, but the dielectric constant approximates that ofMylar®, approximately 3ε_(o). The thickness is approximately 6 mils.

Referring now to FIG. 28, there is illustrated a simulated voltageversus time plot for a sheet of paper as it travels around the drum fourtimes during a four pass color print. The attach and transfer voltagetransition shown in the center of the figure are for a single page of amulti-page print job. The voltage available for paper attach or tonertransfer is the difference between the voltage at the surface of thepaper and ground potential. In FIG. 28, it can be noted that the voltageavailable for paper attach is dependent on the voltage left on the filmlayer by the previous (and fourth toner layer) transfer. As a result,subsequent pages of a multi-page print job will not be gripped as firmlyas the first page. This situation is remedied as illustrated in FIG. 29by applying a discharge voltage with the relay 216 labelled K_(F) to theupper surface of the film layer 194. The voltage is approximately 1500volts in the attach operation in the nip 200 whereas the attach voltagein FIG. 28 is less than 750 volts.

Referring now to FIG. 30, there is illustrated a side-view of theoverall electrophotographic printer mechanism depicting an embodiment ofthe present invention utilizing a buried electrode drum 48 whichutilizes a single electrode or multiple electrodes and the grippinglayer described hereinabove with respect to FIGS. 10, et seq. The paperis fed from a paper tray 238 into an inlet paper path 240. Further, itcan be routed from a manual exterior paper path 242. The paper is thenrouted between two rollers, a lower roller 244 and an upper roller 246,which provide a "pre-curl" operation, which will be described in moredetail hereinbelow. The paper is then fed into the nip 200 between theattached electrode roller 198 and the drum 48, as described above.

After the multiple images have been disposed on the paper for a colorprint, or a single image has been disposed on the paper for a black andwhite print, a stripper arm 248 is provided that is operable to rotatedown about a pivot point 250 onto the surface of the drum 48 to extractor "strip" the paper from the surface of the drum 48, since the paper iselectrostatically held to the drum 48. For multiple prints, the stripperarm 248 is rotated up away from the drum and the attach electrode roller198 is also pulled away from the drum during the multiple passes.

A cleaning roller 254 is provided which can be lowered onto the surfaceof the drum 48 for a cleaning operation after the paper has beenstripped therefrom and prior to a new sheet being disposed thereon.Although not illustrated, a brush or roller similar to the roller 40 ofFIG. 6A is utilized to supply voltage to the electrode layer.

The rollers 244 and 246, as will be described in more detailhereinbelow, are utilized to place a "pre-curl" on the paper such thatit curves upwards about the drum 48. This significantly lowers thevoltage required in order to attach the paper with the attach electroderoller 198. If this is not utilized, a significantly higher voltage isrequired to properly grip paper or the paper will slip. It is necessaryfor the paper to go around at least one revolution before the paperrelaxes onto the drum in the appropriate shape, after which the voltagecould be lowered. However, by pre-curling the paper with the rollers 244and 246, this is alleviated. This pre-curl operation is achieved byusing slightly different durometers for the rollers 244 and 246.

The fuser 100 incorporates two rollers 256 and 258, the roller 258 beingthe heated roller and the roller 256 being the mating roller to form anip therebetween. When the stripper arm 248 strips the paper off of thesurface of the drum 248, this paper is routed into the nip between therollers 258 and 256. The durometers of the rollers 258 and 256 areselected such that the roller 256 is softer than the roller 258 and suchthat the paper will tend to curl around the roller 258, thus providing a"de-curl" to the paper to allow the paper to again flatten out. Thedurometer of the roller 256 is approximately 30 mms and the durometer ofthe roller 258 is approximately 40 mms. The paper is then forwarded toeither a transfer path 260 or a transfer path 262. The transfer path 260feeds to the nip between two rollers 264 and 266 for output onto theplatform 118. The paper path 262 is routed to the nip between tworollers 268 and 270 for output to an external tray. In addition, as iswell known in the art, the paper will tend to curl toward the surface ofthe fused toner, which is opposite the precurl direction. Therefore,fuser roller durometer need not fully compensate for the precurloperation.

As shown in FIG. 30, toner module 72 is the three color modulecontaining all the required components for development of the colorelectrostatic latent image on the photoconductor. It is shown as asingle inseparable unit to facilitate user handling and is separate fromthe black module 78, so that the black materials can be handledidentically to a black and white only print engine. Furthermore, thecolor module uses a mechanism to withdraw the developer brush such thatthe entire unit: does not need to be moved, thereby reducing the spaceand power required to operate the unit.

Referring now to FIG. 31, there is illustrated a detail of the pre-curlsystem. A bracket (not shown) is operable to hold a pivot pin 272 aboutwhich a pivoting arm 274 pivots. The arm 274 has attached to a distalend thereof the attach electrode roller 198, with a protruding portion276 on the diametrically opposite side of the pin 272 from the electroderoller 198 operable to interface with a cam 278. The cam 278 is operableto pivot about a fixed pivot point 280 on the bracket (not shown) topivot the arm 274.

The arm 274 is operable to be pivoted into two positions, a firstposition wherein the attach electrode roller 198 contacts the drum 48,and the second position (shown in phantom line) which pulls the attachelectrode roller 198 away from the drum. A discharge electrode 284 ispivoted about a pivot pin 286 and has an electrode brush 288 disposed onone end thereof. The discharge electrode 284 is operable to pivot in oneposition such that the electrode brush 288 contacts the surface of thedrum 248 to provide a discharge operation prior to the surface of thedrum rotating into contact with the nip 200 and, in the second position,to be pivoted away from the surface of the drum 48. The protrusion 290on the rear portion of the electrode 284 is operable to interface withthe protrusion 276 on the pivoting arm 274. The discharge electrode 284is spring-loaded (not shown) such that it is biased toward the surfaceof the drum 48 to contact the drum 48, such that when the pivoting arm274 pivots to move the protrusion 276 away from the protrusion 290, theelectrode brush 288 will pivot into contact with the drum 48. When thepivoting arm 274 pivots counterclockwise to move the attach electrode198 away from the surface of the drum 48, the protrusion 276 urges theprotrusion 290 up and pivots the electrode 284 and the electrode brush288 away from the surface of the drum 48. The discharge electrode 288 isconnected to the same attach/transfer voltage supply, a supply 294, thatthe buried electrode layer of drum 48 is connected to.

The paper is fed into a paper path 296, which paper path is comprised oftwo narrowing flat surfaces that direct the paper. The paper is directedto a nip 298 between the rollers 244 and 246. The roller 246 pivotsabout the pivot pin 272 and the roller 242 pivots about a slidable pin300. The pin 300 slides in a slot 302 which is disposed in the bracket(not shown). The roller 244 has a durometer that is softer than thedurometer of the soft roller 246 such that the paper will tend to rollaround the roller 246. The size of the rollers 244 and 246 can beselected to determine the amount of pre-curl required. Further, thedurometers of the two rollers 244 and 246 can also be selected in orderto accommodate various thicknesses and weights of paper. In oneembodiment, the durometer of roller 244 is 20 mms, and the roller 246 isa rigid material such as steel. As such, a given size relationshipbetween the rollers 244 and 246 and a given durometer relationshiptherebetween for a set force therebetween will not necessarily insurethe appropriate pre-cud. If the attachment voltage on the drum 48 isreduced to as low a level as possible, this pre-curl adjustment may becritical to insure that the paper adequately adheres to the surface ofthe drum 48 for all weights of paper. To facilitate an adjustment tothis, the roller 244 has a collar 304 disposed on one end thereof thatis rotatable with the roller 244 about pivot pin 300 and the collar 304interacts with a lever 306. Lever 306 is pivoted at one end to a fixedpivot pin 308 and, at the other end, rests on the end of a piston 310.The piston 310 has a threaded end on the opposite end from the lever 306which is threadedly engaged with a nut 310 that is secured in the frame.An adjustment wheel 312 is disposed about the piston 310 to allow handadjustment thereof. In this manner, the pin 300 can be reciprocatedwithin the slot 302. It should be noted that the pin 300 is biaseddownward against the lever by a spring attachment (not shown).

Referring now to FIG. 31A, there is illustrated a detail of the pre-curloperation for the rollers 244 and 246. It can be seen that the paper ispre-curled by the deformation of the roller 244 such that the paperretains a memory of the curling operation. Thus, when the paper is fedto the attach nip 200, the paper will exhibit less of a normal forcedirected away from the surface of the drum 48.

As shown in FIGS. 30 and 31, a mechanism comprised of a conductive rollis employed to urge the paper against the BED surface. Although this isthe preferred embodiment, it is envisioned that a lower cost alternativewould be to use the photoconductor itself as the initial member to urgethe paper against the BED surface. This would eliminate the need for themoving member 274 as shown in FIG. 31.

It has been noted that in order to grip paper to a drum or curvedsurface electrostatically, that the electrostatic gripping forces mustbe sufficient to overcome the inherent stiffness of the paper.Specifically, the greater the stiffness of the paper, the higher is theelectrostatic gripping force and associated voltage to achieve thatforce. In order to use a single voltage to transfer and grip, thegripping voltage must be reduced for stiffer papers so that the transfervoltage exceeds the minimum voltage threshold for gripping.

Numerous papers have been tested to determine their inherent stiffnessand ability to be permanently curled in a hard/soft roller combination.As a result of this testing, it has been determined that there is aminimum threshold of paper deflection that must occur in a precurlsystem to ensure all materials will be adequately gripped onto the drum.Furthermore, in order to minimize unnecessary curl in paper, thisthreshold can be adjusted by a predetermined amount and still achievesatisfactory gripping.

FIG. 32a shows a method to measure the permanent cud or set that occursin paper after it has been run through the precurling apparatus as shownin FIG. 33. The angle of curl (Θ_(c)) is used to determine the paper'scuff characteristic. It was determined by measuring the height off aflat surface that the precurled paper rises. Conversely, some papers areinherently very flexible and do not require precurling to reduce theelectrostatic gripping force. FIG. 32b shows a method to measure thestiffness (or flexibility) of the paper. In this method, the paper isallowed to droop unsupported over a fixed length and the angle of repose(droop angle) is measured (Θ_(d)).

If these angles are summed, then a figure of merit, M, is provided forpaper where the value of M increases for papers that are easier to gripand require less precurl. The figure of merit, "M", is the sum of thepaper's stiffness ("Droop Angle", Θ_(d)) and its ability to be curled("Curl Angle", Θ_(c)): ##EQU3## Where k is a constant value determinedto "normalize" a standard paper. The values Y_(c), X_(c), Y_(d), andX_(d) are determined from measurements taken from the curl and droopexperiments.

Table 3 shows a chart of popular paper types in order of figure ofmerit. The figure of merit has been normalized to a value of 10 for awidely used paper type in laser printers. Tables 4 and 5 illustrateresults of curl and droop experiments for the assortment of papers.

                  TABLE 3                                                         ______________________________________                                                   Curl      Droop                                                             Weight  Y.sub.c X.sub.c                                                                             Y.sub.d                                                                             X.sub.d                                  Paper Type                                                                             (lb.)   (mm)    (mm)  (mm)  (mm)  M                                  ______________________________________                                        Paper Type 1                                                                           28      10.0    48.4  7.5   79.0  8.0                                Paper Type 2                                                                           20      9.3     46.8  9.5   78.0  8.5                                Paper Type 3                                                                           24      12.3    47.8  9.5   78.0  10.0                               Paper Type 4                                                                           21      12.7    49.6  9.5   78.0  10.0                               Paper Type 5                                                                           20      3.9     24.6  18.5  76.5  10.6                               Paper Type 6                                                                           18      12.6    53.8  15.0  77.0  11.3                               Paper Type 7                                                                           20      17.0    51.4  10.0  78.0  12.1                               Paper Type 8                                                                           18      1.7     12.4  27.5  74.0  13.4                               Paper Type 9                                                                           13      1.6     16.2  31.0  73.0  13.8                               ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Large Roller Radius, R (mm):                                                                    12.5   12.5   12.5 12.5 12.5                                Small Roller Radius, r (mm):                                                                    5.0    5.0    5.0  5.0  5.0                                 Roller Interference, d (mm):                                                                    0.5    1.0    1.5  2.0  2.5                                 Center-to-Center Dist, D (mm):                                                                  17.0   16.5   16.0 15.5 15.0                                Nip Angle, theta (deg):                                                                         8.6    12.0   14.5 16.5 18.2                                Nip Width, S (mm):                                                                              1.9    2.7    3.4  4.0  4.5                                 ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                                   Curl Angle + Droop Angle (deg)                                     ______________________________________                                        theta/r (deg/mm):                                                                        1.7       2.4    2.9    3.3  3.6                                   Paper Type                                                                    Paper Type 1                                                                             5.4       12.0   17.1   20.3 23.3                                  Paper Type 2                                                                             11.4      18.1   18.2   21.0 22.3                                  Paper Type 3                                                                             10.2      14.8   21.4   24.1 24.1                                  Paper Type 4                                                                             11.5      13.8   21.3   23.4 24.1                                  Paper Type 5                                                                             23.6      21.3   22.6   22.8 22.6                                  Paper Type 6                                                                             18.5      20.3   24.2   25.1 25.3                                  Paper Type 7                                                                             10.9      19.0   25.6   27.1 26.7                                  Paper Type 8                                                                             26.0      27.1   28.2   28.1 27.5                                  Paper Type 9                                                                             29.4      29.3   28.6   29.6 30.6                                  ______________________________________                                    

FIG. 33 illustrates the precurl configuration of a soft roller 300 andhard roller 302 that deflects paper through a subtended angle Θ (nipangle). The radius of curvature, r, of the hard roller along with thenip angle, Θ, as caused by the interference with the soft roller radius,R, determines the amount of curl. Tables 4 and 5 illustrate the resultof the precurl function combined with the stiffness of the paper versusthe nip angle by radius of curvature quotient for various paper types.It is interesting to note that the some materials show little change asa function of Θ/r. This is due to the fact that these materials areobserved to be very flexible and require no precurl to grip, (i.e., theyare always above the threshold). Of particular interest is the fact thatfor good performance for all paper types tested a minimum threshold of2.9 degrees per millimeter or 15 degrees curl plus droop angle isrequired. If it is desired to reduce or increase the amount of cud fordifferent media then the appropriate Θ/r can be determined by selectingthe curl droop angle sum to be above 15 degrees.

It should be noted that the threshold of curl plus droop may increase tothe fourth power of the proportionately to the decrease of the radius ofcurvature. For example, the gripping threshold for a drum radius of 65millimeters (the above threshold is for 70 millimeters) would increaseby 34% (or (70/65)⁴) to 20 degrees (3.3 degrees/mm for the stiffestmaterial tested).

Although the preferred embodiment has been described in detail, itshould be understood that various changes, substitutions and alterationscan be made therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A print engine paper feed device for feedingpaper onto a rotating arcuate surface, comprising:a directing device fordirecting a sheet of paper along a defined path; a precurl device fordeforming said sheet of paper to have an arcuate deformation that allowssaid sheet of paper to follow an arcuate path in the direction ofcurvature of the rotating arcuate surface defined as a predeterminednumber of degrees of path curvature per path millimeter of travel thatis greater than a predetermined minimum and a combined curl-droop anglegreater than a predetermined minimum, said curl-droop angle defined asthe sum of the angle of repose of said sheet of paper prior todeformation thereof over an unsupported fixed length and the angle ofcurl of said sheet of paper over a fixed length after deformationthereof; a precurl control for controlling the amount of arcuatedeformation imparted to said sheet of paper by said precurl device; andan attachment device for attaching said sheet of paper to the rotatingarcuate surface after arcuate deformation thereof by said precurldevice.
 2. The print engine paper feed device of claim 1, wherein therotating arcuate surface has a predetermined curvature associatedtherewith and said arcuate deformation corresponds to the direction ofcurvature of the rotating arcuate surface.
 3. The print engine paperfeed device of claim 1, wherein said precurl device comprises:a firstroller with a first durometer; a second roller with a second durometerdisposed adjacent said first roller to form a nip therebetween with apredetermined pressure between said first roller and said second rollerat said nip and a predetermined nip angle theta; the durometer of saidfirst roller greater than the durometer of said second roller such thatsaid second roller will be deformed at said nip; and at least one ofsaid second rollers being driven.
 4. The print engine paper feed deviceof claim 3, wherein said precurl control comprises a variable pressuredevice for varying the predetermined pressure at said nip to define thedeformation of said second roller with the paper disposed in said nip.5. The print engine paper feed device of claim 1, wherein said firstroller has substantially no deformation associated therewith due to thepredetermined pressure at said nip.
 6. The print engine paper feeddevice of claim 1 wherein said curl-droop angle is greater than 15°. 7.A method for feeding paper onto a rotating arcuate surface in a primengine, comprising the steps of:directing a sheet of paper along adefined path; deforming the sheet of paper to have an arcuatedeformation that allows the sheet of paper to follow an arcuate path inthe direction of curvature of the rotating arcuate surface defined as apredetermined number of degrees of path curvature per path millimeter oftravel that is greater than a predetermined minimum and a combinedcurl-droop angle greater than a predetermined minimum, the curl-droopangle defined as the sum of the angle of repose of the sheet of paperprior to deformation thereof over an unsupported fixed length and theangle of curl of the sheet of paper over a fixed length afterdeformation thereof; controlling the amount of arcuate deformationimparted to the sheet of paper in the step of deforming; and attachingthe paper to the rotating arcuate surface after arcuate deformationthereof by the step of deforming.
 8. The method of claim 7, wherein therotating arcuate surface has a predetermined curvature associatedtherewith in a predetermined direction and the step of deformingoperable to impart an arcuate deformation to the paper that correspondsto the direction of curvature of the rotating arcuate surface.
 9. Themethod of claim 7, wherein the step of deforming the sheet of papercomprises:providing a first roller with a first durometer; providing asecond roller with a second durometer; disposing the first rolleradjacent the second roller at a predetermined compression therebetweento form a nip therebetween, the nip disposed along the defined path andhaving a predetermined nip angle theta; the durometer of the firstroller greater than the durometer of the second roller such that thesecond roller will deform at the nip; and driving at least one of therollers.
 10. The method of claim 9, wherein the step of controlling theamount of arcuate deformation provided by the step of deformingcomprises supplying a variable pressure to at least one of the first andsecond rollers to vary pressure at the nip to define the deformation ofthe roller with the paper disposed at the nip.
 11. The method of claim9, wherein the first roller has substantially no deformation associatedtherewith due to the predetermined pressure at the nip.
 12. A printengine paper feed device for feeding paper onto a rotating arcuatesurface of radius R millimeters, comprising:a directing device fordirecting a sheet of paper along a defined path; a precurl device fordeforming said sheet of paper to have an arcuate deformation along anarcuate path that is equal to or exceeds 2.9 degrees of path curvatureper path millimeter of paper travel; an attachment device for attachingthe paper to the rotating arcuate surface after arcuate deformationthereof by said precurl device.
 13. A print engine paper feed device forfeeding paper onto a rotating arcuate surface, comprising:a directingdevice for directing a sheet of paper along a defined path; a precurldevice for deforming said sheet of paper to have an arcuate deformationthat allows said sheet of paper to follow an arcuate path in thedirection of curvature of the rotating arcuate surface defined as acombined curl-droop angle greater than 15°, said curl-droop angledefined as the sum of the angle of repose of said sheet of paper priorto deformation thereof over an unsupported fixed length and the angle ofcurl of said sheet of paper over a fixed length after deformationthereof; a precurl control for controlling the amount of arcuatedeformation imparted to said sheet of paper by said precurl device; andan attachment device for attaching said sheet of paper to the rotatingarcuate surface after arcuate deformation thereof by said precurldevice.